Fiber-reinforced resin materials, or “composite” materials as they are commonly known, have relatively high strength-to-weight ratios, good corrosion resistance, and other beneficial properties that make them particularly well suited for use in aerospace applications. Conventional composite materials typically include glass, carbon, or polyaramid fibers in woven and non-woven configurations. In the raw material stage, the fibers can be formed into tapes, filaments, and fabric sheets that are pre-impregnated with uncured resin. The raw materials can be manufactured into parts by laminating them onto a mold surface, and then applying heat and pressure to cure the resin and harden the laminate. Composite sandwich structures can be manufactured by laminating a core material (e.g., a foam or honeycomb material) between two facesheets composed of laminated plies, tapes, and/or filaments. Facesheets can also include one or more metallic layers.
Because of their relatively high strength-to-weight ratios, composite materials are often used in aircraft structures to reduce weight and increase performance. In fighter aircraft, business jets, and other relatively high-performance aircraft, for example, composite materials have been used in both primary and secondary structures. In large commercial aircraft, however, the use of composite materials has traditionally been limited to non-critical, secondary structures, while wing spars and other primary structures have been manufactured predominantly from metals such as aluminum, titanium, etc.
When used in primary structure, composite wing spars are typically manufactured by forming a solid laminate of fiber plies having a “C” cross-sectional shape. This relatively simple method reduces part count and lends itself well to automated lay-up procedures. One downside of this approach, however, is that it can be difficult to vary the ply count over the length and height of the spar. As a result, some portions of the spar may be much thicker (and heavier) than they need to be to meet localized structural requirements. In addition, composite spars manufactured in this way often have to be reinforced with stiffeners which are bolted or bonded to the spar web between ribs to limit buckling. Moreover, such spars often do not meet ground plane and electromagnetic effects (EME) requirements without the addition of relatively heavy ground cables to the upper and lower portions of the spar.